People have had soft comfortable shoes reserved for indoor wear pretty much since shoes were first invented; if only because outdoor shoes got muddy and in Europe, unlike the Middle East, it was often too cold to pad about barefoot indoors. The Romans had a kind of soft-soled slip-on shoe called a "soccus", the origin of our word "sock". (In fact they had socks as well, but they called them "udones".) It is a safe bet that in the Roman province of Britannia people wore them at home.
Indoor shoes in medieval Britain were often the same shape as their outdoor shoes (so in contemporary illustrations you can't tell the difference), only made in softer materials. Really luxurious indoor shoes for winter were fur-lined. The first known sighting of the specific word "slipper" for a slip-on indoor shoe is from the late 15th century.
From then on, slippers are *mentioned* quite often in historical letters, inventories and literature. But they did not get shown much in art, because people having their portraits painted usually put on their most formal suit for the purpose. But at the beginning of the 18th century a trend began of having oneself painted in casual indoor clothes, and in that kind of portrait you very often see slippers, as in the source below.